The present invention relates to a method for determining a preceding wafer to be used for preceding processing among a plurality of semiconductor wafers constituting one lot, a method for determining a measuring wafer to be used for measurement of processing results among a plurality of wafers constituting one lot, and a method for adjusting the number of semiconductor wafers constituting one lot.
In fabrication of semiconductor devices, semiconductor wafers that are grouped into lots are subjected to a plurality of processes every lot. In general, the performance of a fabrication apparatus used in a process varies with lots. The processing results therefore vary with lots even when all lots are subjected to the same process under the same processing conditions.
In order to suppress a variation in processing results among lots, preceding processing is adopted. That is, among a plurality of semiconductor wafers constituting one lot (hereinafter, referred to as inner-lot wafers), one to several semiconductor wafer(s) are selected as a preceding wafer or wafers. A given process is preliminarily performed for the preceding wafer(s). Based on the results of the given process on the preceding wafer(s), the conditions of the given process are adjusted, so that the other semiconductor wafers in the same lot are subjected to the given process under the adjusted conditions.
In fabrication of semiconductor devices, also, a measuring process is inserted during the fabrication to examine whether or not the results of a process already performed for a lot have reached the target value for the process. This measuring process is generally performed for one to several measuring wafer(s) selected among the inner-lot semiconductor wafers for improvement of the throughput.
Conventionally, as the preceding wafer or the measuring wafer, selected is a semiconductor wafer located outermost in a wafer box among inner-lot wafers, or a semiconductor wafer having the least wafer number (wafer number is given uniquely to each semiconductor wafer) or a heading wafer identification (ID) (ID is given uniquely to each semiconductor wafer).
That is, in the conventional methods, the preceding wafer or the measuring wafer is determined without consideration of a variation in processing results among inner-lot wafers. This causes problems as described below.
(Problem of Conventional Method for Determining Preceding Wafer)
A semiconductor wafer determined as a preceding wafer may be a wafer having processing results largely deviated from the average of the processing results of a process performed for wafers in the same lot prior to a given process for which preceding processing is intended. In such a case, if the conditions of the given process are adjusted based on the processing results of the given process on the preceding wafer and the given process is performed for the inner-lot wafers other than the preceding wafer under the adjusted conditions, the processing results of the given process may fail to be within specifications for a majority of the inner-lot wafers.
The above problem of the conventional preceding wafer determination method will be described with reference to FIGS. 9(a) and 9(b), taking the case of performing preceding processing for an etching process as an example. The etching in this case is made for a film deposited on each of five semiconductor wafers constituting one lot. In FIGS. 9(a) and 9(b), the respective semiconductor wafers as substrates are omitted for simplification.
FIG. 9(a) illustrates the results of the process of depositing films on semiconductor wafers Nos. 1 to 5 (film deposition process). As shown in FIG. 9(a), among the thicknesses of the deposited films (deposited film thicknesses) of semiconductor wafers Nos. 1 to 5, denoted by d1, d2, d3, d4, and d5, respectively, the deposited film thickness d1 of semiconductor wafer No. 1 is smallest.
FIG. 9(b) illustrates the results of the etching process performed for semiconductor wafers Nos. 1 to 5 after the film deposition process based on preceding processing performed for semiconductor wafer No. 1 as a preceding wafer. More specifically, semiconductor wafer No. 1 is preliminarily etched so that the film thickness remaining after the etching (post-etching film thickness) is equal to a predetermined target value (target film thickness), to determine the etching time required for the etching process. Thereafter, remaining semiconductor wafers Nos. 2 to 5 are etched for the determined etching time. At this time, it is presumed that the etching amounts, that is, the etching depths of the films on semiconductor wafers Nos. 1 to 5 are roughly the same.
As shown in FIG. 9(b), the post-etching film thicknesses of semiconductor wafers Nos. 2 to 5 are larger than the target film thickness by e2, e3, e4, and e5, respectively. In particular, the post-etching film thicknesses of semiconductor wafers Nos. 2, 4, and 5 exceed the upper limit. In FIG. 9(b), the respective areas defined by the dotted lines represent the portions of the films deposited at the film deposition process shown in FIG. 9(a) but removed at the etching process.
(First Problem of Conventional Method for Determining Measuring Wafer)
In measurement of the processing results of a given process on inner-lot wafers using a measuring wafer, the semiconductor wafer determined as the measuring wafer may be a wafer having the processing results largely deviated from the average of the processing results of a process performed for wafers in the same lot prior to the given process. In such a case, the average processing results of the given process on the inner-lot wafers will not be obtained by measuring the processing results of the given process on the measuring wafer.
The above first problem of the conventional measuring wafer determining method will be described with reference to FIGS. 10(a) and 10(b), taking as an example the case of measuring the film thickness remaining after etching (post-etching film thickness) of a film deposited on each of five semiconductor wafers Nos. 1 to 5 constituting one lot. In FIGS. 10(a) and 10(b), the respective semiconductor wafers as substrates are omitted for simplification.
FIG. 10(a) illustrates the results of the process of depositing films on semiconductor wafers Nos. 1 to 5 (film deposition process). As shown in FIG. 10(a), among the deposited film thicknesses of semiconductor wafers Nos. 1 to 5, denoted by s1, s2, s3, s4, and s5, respectively, the deposited film thickness s1 of semiconductor wafer No. 1 is most largely deviated from a predetermined target value (target deposited film thickness).
FIG. 10(b) illustrates the results of the etching process performed for semiconductor wafers Nos. 1 to 5 after the film deposition process. Assume that the etching process has been performed for semiconductor wafers Nos. 1 to 5 for the same etching time calculated based on the difference between the target deposited film thickness in the film deposition process and the target film thickness in the etching process. At this time, it is presumed that the etching amounts, that is, the etching depths of the films on semiconductor wafers Nos. 1 to 5 are roughly the same. In this case, as shown in FIG. 10(b), the post-etching film thickness t1 of semiconductor wafer No. 1 is most largely deviated from the average of the post-etching thicknesses t1, t2, t3, t4, and t5 of semiconductor wafers Nos. 1 to 5. Therefore, by measuring the post-etching film thickness of semiconductor wafer No. 1, having the least wafer number, as a measuring wafer, the average post-etching film thickness of semiconductor wafers Nos. 1 to 5 will not be obtained.
(Second Problem of Conventional Method for Determining Measuring Wafer)
Another problem arises when the processing results of a given process on inner-lot wafers are measured using a measuring wafer and the measuring wafer used must be discarded. The measuring wafer may be a semiconductor wafer having processing results close to the target value of the processing results of a process performed for the inner-lot wafers prior to the given process, that is, a semiconductor wafer having good processing results. This results in discarding the semiconductor wafer having good processing results.
The above second problem of the conventional measuring wafer determining method will be described with reference to FIGS. 11(a) and 11(b), taking as an example the case of measuring steps on the surfaces of five semiconductor wafers Nos. 1 to 5 constituting one lot after multi-layer wiring structures have been formed thereon. The measurement of steps on the semiconductor wafer surface (step measurement) includes determining whether or not steps within specifications have been formed for the film and the like on the semiconductor wafer by etching or the like. This is performed by physically tracing the surface of the semiconductor wafer with a fine needle or the like, for example. The semiconductor wafer used for this step measurement therefore must be discarded without being used for subsequent processes.
FIG. 11(a) illustrates the results of a process of forming lower lines on semiconductor wafers Nos. 1 to 5 (lower line formation process). Referring to FIG. 11(a), a lower line 2 made of an aluminum film, for example, is formed on each of semiconductor wafers Nos. 1 to 5 as a substrate 1. The electric characteristic data, for example, the electric resistances of the lower lines 2 of semiconductor wafers Nos. 1 to 5 (lower line resistances), denoted by r1, r2, r3, r4, and r5, respectively, are measured after the lower line formation process. Assume that the lower line resistance r1 of semiconductor wafer No. 1 is closest to the target value of the lower line resistance. That is, the electric characteristic data of the lower line 2 of semiconductor wafer No. 1 is better than those of the lower lines 2 of semiconductor wafers Nos. 2 to 5.
FIG. 11(b) illustrates the results of a process of forming upper lines on semiconductor wafers Nos. 1 to 5 (upper line formation process). Referring to FIG. 11(b), an interlayer insulating film 3 is formed over each of semiconductor wafers Nos. 1 to 5 as the substrate 1 including the lower line 2 formed thereon, and an upper line 4 made of an aluminum film, for example, is formed on the interlayer insulating film 3. The upper line 4 is formed by etching an aluminum film or the like deposited on the interlayer insulating film 3.
If semiconductor wafer No. 1, having the least wafer number, is used as a measuring wafer for the step measurement, semiconductor wafer No. 1 including the lower line 2 having the best electric characteristic data will be discarded after the step measurement.
(Other Problem)
Conventionally, in order to avoid excessive production of semiconductor devices as products, the number of lots newly fed (lots introduced into a fabrication apparatus used for an initial process) is adjusted, or the number of semiconductor wafers in a lot newly fed is adjusted, based on the yield of a lot that has completed all the processes.
The conventional method for adjusting the number of wafers has the following problem. The yield may be under enhancement while a lot is under processing. Since all wafers in a lot already fed follow a predetermined process procedure, yield enhancement may results in excessive production of semiconductor devices as products. If the inner-lot wafers are subjected to a process using a single wafer processing equipment, the turn around time (TAT) increases unnecessarily.